At the transition of an electric field from a first medium to a second medium, electric stresses harmful to the electric equipment may arise due to the resulting electric field. In a shielded high-voltage cable for instance, the electric field is uniform along the cable axis and there is a variation in the field only in the radial direction. When the cable is terminated or spliced, the shield of the cable is removed for a distance along the cable. The removal of the shield causes a discontinuity in the electric field at the shield end, resulting in high electric stresses. These high stresses must be reduced in order not to impair the expected life of the system.
The electric stresses in question can be reduced by grading the electric field at the transition of the electric field from the first medium to the second medium, e.g. from a shielded cable part to a cable part where the original shield has been removed. A number of methods have been developed and employed for this kind of field grading. The present invention is related to so called resistive and capacitive field grading.
The resistive field grading can be used in alternating current as well as direct current application. Resistive field grading may also be used in order to achieve field grading when voltages are occurring in the form of impulses. In case of a cable ending of the kind indicated above, a body having a suitable resistance is introduced around the unshielded part of the cable in the area closest to the shielded part of the cable and in electric contact with the shield. When a voltage is applied across the cable a current flows through the body towards the shield of the cable, which shield is at earth potential. A resistive voltage drop then occurs in the body, resulting in a more uniform distribution of the potential. This potential distribution will be more linear if the body consists of a material exhibiting a non-linear electrical resistance that decreases with an increasing electric field. The closer to the edge of the shield, the higher the electric field will be in the field grading body and, consequently, the lower the electrical resistance in the body if the body exhibits such a non-linear electrical resistance. In this way, the voltage drop along the field grading body will become more uniformly distributed in a body that exhibits such a non-linear electrical resistance than in a body that does not.
The capacitive field grading is used in alternating current applications. However, capacitive field grading may also be used in order to achieve field grading when voltages are occurring in the form of impulses. In case of a cable ending of the kind indicated above, a body of a material having a dielectric constant higher than that of the insulation and as low losses as possible is introduced around the unshielded part of the cable in the area closest to the shielded part of the cable and in electric contact with the shield, whereby a spreading of the equipotential lines will be achieved. Capacitive filed grading properties are also desired in a material adapted for grading the electric field in high voltage direct current applications so as to achieve an effective field grading in case of suddenly occurring voltage surges.
The field grading material used in field grading bodies of prior art comprises a dielectric matrix, normally a polymer, and a plurality of micro varistor particles evenly distributed in said matrix. The micro varistor particles comprise a material the distinguishing property of which is its electrical resistivity, which is to a large extent dependent of the strength of an electric field applied thereto. The dependency is non-linear. Typical micro varistor particle materials are materials such as SiC or ZnO. Normally, said micro varistor particles are of spherical shape with a mean diameter size in the range of 30-100 μm. Typically, they occupy approximately 20-25% of the volume of the field grading material.
However, in order to fulfil their task of conducting an electrical current and thereby equalising an electrical field, the micro varistor particles must be added to the matrix material in such an amount that they will heavily impact the mechanical properties of the field grading material, making it more rigid, more brittle and less readily workable into the shape needed for a specific application.
In order to solve this problem, prior art suggests the incorporation of a plurality further particles, of electrically conducting character, but having, less detrimental effect on the mechanical properties of the field grading material than has the micro varistor particles. Prior art thereby suggests the use of carbon black as such further particles. By the introduction of such further electrically conducting particles the resistivity of which is to a much lesser degree dependent of the strength of an applied electric field, the concentration of the micro varistor particles may be reduced, since the carbon black particles, if added to a sufficient amount, will form electrically conducting bridges between individual micro varistor particles.
However, the carbon black particles will also induce an increase of the electrical conductivity to a piece of field grading material in the non-excited condition thereof, i.e. when there is no electric field applied thereto or only a weak electric field thereto. Such conductivity will result in losses and is, accordingly, of disadvantage for the total efficiency of an insulation arrangement using a field grading body made of such a field grading material.